EENS 2120 Petrology Prof. Stephen A. Nelson Igneous Rocks of the Ocean Basins The Ocean Basins The ocean basins cover the largest area of the Earth's surface. Because of plate tectonics, however, most oceanic lithosphere eventually is subducted. Thus the only existing oceanic lithosphere is younger than about Jurassic in age and occurs at locations farthest from the oceanic spreading centers. Except in areas where magmatism is intense enough to build volcanic structures above sea level, most of the oceanic magmatism is difficult to access. Samples of rocks can be obtained from drilling, dredging, and expeditions of small submarines to the ocean floor. Numerous samples have been recovered and studied using these methods. Most of the magmatism is basaltic. Still, few drilling expeditions have penetrated through the sediment cover and into the oceanic lithosphere. Nevertheless, we have a fairly good understanding of the structure of the oceanic lithosphere from seismic studies and ophiolites. Here we will first look at ophiolites, then discuss basaltic magmatism in general, and then discuss the various oceanic environments where magmatic activity has occurred. Ophiolites An ophiolite is a sequence of rocks that appears to represent a section through oceanic crust. Ophiolites occur in areas where obduction (the opposite of subduction) has pushed a section of oceanic lithosphere onto continental crust. During this process, most ophiolite sequences have been highly deformed and hydrothermally altered. Nevertheless, it is often possible to look through the deformation and alteration and learn something about the structure of oceanic lithosphere. An idealized ophiolite sequence shows an upper layer consisting of deep sea sediments (limestones, cherts, and shales), overlying a layer of pillow basalts. Pillow basalts have a structure consisting of overlapping pillow-shaped pods of basalt. Such pillow structure is typical of lavas erupted under water. The pillow basalts overly a layer consisting of numerous dikes, some of which were feeder dikes for the overlying basalts. Beneath the sheeted dike complex are gabbros that likely represent the magma chambers for the basalts. The upper gabbros are massive while the lower gabbros show layering that might have resulted from crystal settling. At the base of the layered gabbros there is a sharp increase in the density of the rocks, and the composition changes to ultramafic rocks. This sharp change in density is correlated with what would be expected at the base of the crust, and is thus referred to as the petrologic moho. At the top of the ultramafic sequence the rock type is harzburgite (Ol + Opx), a rock type expected to be the residual left from partially melting peridotite. The base of the ultramafic layer is composed of peridotite. Because most ophiolites have been hydrothermally altered, most of the mafic rocks have been altered to serpentinite. Note that ophiolite means "snake rock". Ocean Basins

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